Generally, the air conditioning system of an automobile is driven by the vehicle engine through an electromagnetic clutch. The air conditioning system is designed to achieve a predetermined air conditioning performance at a predetermined air conditioning load when the automobile is driven at an average speed. Thus, when the vehicle engine is idling or is being driven at low speeds, the rotational speed of the compressor is correspondingly low. Therefore, the performance of the air conditioning system is adversely effected. On the other hand, when the vehicle is driven at high speeds, the rotational speed of the compressor is to high for efficient performance. Thus, electromagnetic clutches are used to control the rotational speed of the compressor under varying drive speeds by intermittently stopping and starting the compressor.
However, there are many problems associated with continuously cycling the clutch on and off. For example, when the engine is driven at high speeds and the capacity of the air conditioning system is large, it is necessary for the electromagnetic clutch to be turned on or off frequently. On the other hand, at low speed or when the vehicle engine is idling, the compressor is not sufficiently driven to maintain the desired temperature in the vehicle.
In order to solve the abovementioned problems, a system which controls the capacity of a compressor by detecting the temperature at the outlet side of the air conditioning system evaporator is proposed in published Japanese Patent Application No. 58-30. In such a system, the performance of the air conditioning system is not directly detected. For example, even though the temperature in the inside of the vehicle may be high, the capacity of the air conditioning system is reduced when the temperature at the outlet side of the evaporator becomes lower than a predetermined temperature. Thus, the capacity of the system is insufficient to cool the vehicle. In addition, when the vehicle is running, the capacity of the air conditioning system is changed frequently, thereby placing great stress and strain on the air conditioning system.